1. Field of the Invention
The present invention relates to producing high-energy ultrashort optical pulses, and, in particular, to the use of Bragg fibers for stretching or compressing these pulses in an ultrashort-pulse laser system. Bragg fibers may also be used in the delay lines of optical amplifiers and for the delivery of high peak power pulses of light.
2. Description of Related Art
a. Ultrashort-Pulse Laser Systems
Ultrafast laser technology has been known and used for over 20 years. Chemists and physicists developed ultrafast lasers for the purpose of measuring extremely fast physical processes such as molecular vibrations, chemical reactions, charge transfer processes, and molecular conformational changes. All these processes take place on the time scales of femtoseconds (fsec, 10−15 sec) to picoseconds (psec, 10−12 sec). Carrier relaxation and thermalization, wavepacket evolution, electron-hole scattering, and countless other processes also occur on these incredibly fast time scales.
Optical science using ultrafast, ultrashort optical pulses has seen remarkable progress over the past decade. While definitions vary, in general “ultrashort” generally refers to optical pulses of a duration less than approximately 10 psec, and this definition is used herein. Numerous applications of ultrashort pulses have been developed that would be otherwise impossible or impractical to implement with other technologies. With ultrashort pulses, researchers have investigated many highly nonlinear processes in atomic, molecular, plasma, and solid-state physics, and accessed previously unexplored states of matter. For example, coherent ultrashort pulses at wavelengths as short as 2.7 nm have been generated through harmonic upconversion.
Many applications of ultrashort-pulse (USP) lasers make use of the very high peak power that each pulse momentarily provides. Although the average power from the laser may be quite moderate and the total energy within a pulse small, the extremely short duration of each pulse yields a peak, nearly instantaneous power that is very large. When these pulses are focused on a tiny spot, the high optical power is sufficient to ablate many materials, making a USP laser a useful tool for micromachining, drilling, and cutting. If needed, the precision of the material removal can even exceed that of the beam focus, by carefully setting the pulse intensity so that only the brightest part of the beam rises above the material ablation threshold. The material ablation threshold is the amount of energy density, or fluence, needed to ablate the material, often on the order of 1 J/cm2. Optical pulses containing a much greater energy density are generally considered to be “high-energy” and this definition is used herein.
Ablation with a USP laser differs from longer duration pulse ablation techniques since most of the energy deposited on the surface by the ultrashort optical pulse is carried away with the ablated material from the machined surface, a process which occurs too rapidly for heat to diffuse into the surrounding non-irradiated material, thus ensuring smooth and precise material removal. For most materials, light pulses having a duration less than approximately 10 psec are capable of this non-thermal ablation when the pulse energy exceeds the ablation threshold of the material. Pulses with durations longer than about 10 psec can also ablate material if the pulse energy is greater than the ablation threshold, but thermal damage to the surrounding non-irradiated regions can occur.
Researchers have demonstrated non-thermal ablation techniques by accurately machining many materials, such as diamond, titanium carbide, and tooth enamel. In one interesting demonstration, USP lasers have been used to slice safely through high explosives; this is possible because the material at the focus is vaporized without raising the temperature of, and detonating, the surrounding material. Surgical applications also abound where ultrashort pulses are especially effective because collateral tissue damage is minimized. For example, researchers at Lawrence Livermore National laboratory have used ultrashort pulses to remove bony intrusions into the spinal column without damaging adjacent nerve tissue. Ophthalmic researchers have shown that USP lasers cut a smoother flap from a cornea than standard knife-based techniques and provide more control of the cut shape and location. There are numerous other applications as well. For the purposes of this invention, the term “ablation” used herein will refer to non-thermal ablation as discussed above and enabled by USP lasers, unless expressly indicated otherwise.
Nearly all high peak-power USP laser systems use the technique of chirped pulse amplification (CPA) to produce short-duration, high-intensity pulses. Optical CPA was proposed by Mourou and others in the 1980s, as an extrapolation from previous CPA techniques used in radar microwave applications. Chirped pulse amplification is used to increase the energy of a short pulse while keeping the peak power of the pulse below a level that can cause damage to the optical amplifier. In this technique, the duration of the pulse is increased by dispersing it temporally as a function of wavelength (a process called “chirping”), thus lowering the peak power of the pulse while maintaining the overall power contained in the pulse. The chirped pulse is then amplified, and then recompressed to significantly re-shorten its duration.
By lengthening the pulse in time, the overall pulse can be efficiently amplified by an optical amplifier gain medium while the peak power levels of the chirped pulse remain below the damage threshold of the optical amplifier. The more a signal can be stretched, the lower the peak power, allowing for the use of either lower peak power amplifiers or more efficient amplifiers, such as semiconductor optical amplifiers. The CPA technique is particularly useful for efficient utilization of solid-state optical gain media with high stored energy densities, where full amplification of a non-chirped short duration pulse is not possible since the peak power of the pulse is above the damage thresholds of the amplifier materials. Techniques for generating ultra-short pulses are described in, e.g., Rulliere, C. (ed.), Femtosecond Laser Pulses, (Springer-Verlag, New York, 1988).
A typical CPA system is illustrated in FIG. 1 and works as follows. Ultrashort light pulses are generated at low pulse energies (typically less than 1 nJ) through the use of a modelocked laser oscillator, or “seed source” 101. These pulses are chirped with a chromatically dispersive system or a “stretcher” 102, which may be as simple as a standard silica optical fiber or a diffraction-grating arrangement. The dispersive system stretches the pulse temporally, increasing its duration by several orders of magnitude from, e.g., a duration under 1 psec to approximately 1 nanoseconds (nsec, 10−9 sec), or three orders of magnitude (1000 times). This decreases the pulse peak power by the same factor, three orders of magnitude in this example, so that the total power contained in the pulse remains approximately constant. Next, the stretched pulse is amplified by one or more stages of optical amplification 103 to increase the energy of the pulse. After amplification, the stretched pulse is compressed by a pulse compressor 104 to a pulse having a duration near the original input pulse duration. Finally, the ultrashort, high energy pulse is delivered to a desired location by some delivery mechanism 105. Graphical representations of the treatment of a single pulse are shown between the elements in FIG. 1 (not to scale).
Typically the compression is done with bulk optical elements involving prism and grating pairs or combinations thereof. Pulse-compression techniques of amplified chirped pulses have been well studied; see, e.g., the diffraction grating compressor discussed in U.S. Pat. No. 5,822,097, issued to Tournois. Pulse compression has also been explored in standard optical fibers, where the nonlinear optical interactions in the fibers are exploited to produce ‘soliton compression’. These soliton techniques rely on the optical non-linearities of the fiber to broaden the pulse spectrum and shorten the pulse duration. Soliton compression is typically used to compress pulses that are of the order of a few picoseconds in duration to sub-picosecond durations and the pulses energies are typically well under a few hundred nanoJoules for compression in optical fibers.
This current state of the art CPA for ultrafast systems is sufficient to satisfy the technical and performance requirements for many research applications of USP laser technology. However, there are some practical problems associated with commercializing applications of USP laser technology that have prevented USP technology from gaining mainstream acceptance.
For example, in standard silica optical fibers, the high peak power of a compressed high-energy pulse increases nonlinear optical effects in the fiber, such as self-phase modulation and stimulated Raman scattering, which distort the pulse and generally prevent pulse re-compression. (Raman scattering shifts the wavelength of a portion of the incoming light to a longer color and thereby separates that energy from the original signal.) Kerr effect nonlinearities, which include self-phase modulation, can cause pulse spectrum breakup, self focusing and catastrophic failure in the fiber. One method of reducing the nonlinear effects in an optical fiber compressor is to increase the effective area of the propagating mode to decrease the peak power in the fiber, but this technique has been limited to producing pulses with a maximum energy of only a few J. FIG. 3a shows an example of pulse degradation due to nonlinearities in a standard large core silica fiber.
There is another problem with using a standard silica optical fiber to stretch the optical signal. The standard dispersion for optical fiber is on the order of 17 psec/nm/km at 1550 nm (picoseconds of delay or temporal stretch per nanometer of wavelength per kilometer of fiber). To get the three orders of magnitude of stretch in the example above would require tens of kilometers of fiber. For this reason, commercial applications are generally only able to stretch a 1 psec pulse to about 200 psec, which does not allow for sufficient amplification for many applications. (It is possible to stretch such a pulse to 1 nsec by the use of a folded bulk grating configuration, but that still requires an internal optical path length of many meters. In addition, the folding process requires difficult optical alignments.)
Compression is also a problem, and the greater the stretching, the more compression is required. While optical gratings can also be used to compress optical signals, there are also limitations on the size of such gratings, thus limiting the amount of compression that can be achieved. Thus, the size of the USP laser system rapidly becomes prohibitive for many applications compared to its performance. For these reasons, in many cases commercial applications have been tabled due to the lack of practical USP laser sources. The need exists for well-packaged, turnkey USP laser systems that are cost effective, robust, and compact.
b. Bragg Fibers
Bragg fibers have been studied since the 1970s, e.g., Yeh, Yariv, & Marom, “Theory of Bragg fiber,” Journal of the Optical Society America 68 (9), pp. 1196 (September 1978), but were not made until the late 1990's, when they became the subject of more recent investigations. See, e.g., Engeness, Ibanescu, Johnson, Weisberg, Skorobogatiy, Jacobs, and Fink, “Dispersion Tailoring, And Compensation By Modal Interactions In Omniguide Fibers,” Optics Express 11 (10), pp. 1175-1196 (19 May 2003) and Fink, Ripin, Fan, Chen, Joannopoulos, & Thomas, “Guiding Optical. Light In Air Using An All-Dielectric Structure,” J of Lightwave Technology 17 (11), pp. 2039-2041 (November 1999). Bragg fibers may also be known as Omniguide fibers, after one manufacturer. Many researchers have studied Bragg fibers for use as links in long-haul fiber optic communications systems, since the fibers were predicted to have very low intrinsic optical losses.
A typical Bragg fiber consists of at least two substantially annular rings surrounding an inner core filled with gas or liquid, each ring having a distinct refractive index. A pair of such substantially annular rings of differing refractive index is commonly referred to as a “bilayer.” In some versions, the rings are formed from alternating regions of high contrast refractive index material, for example alternating layers of high and low dielectric materials. A radial light ray from the fiber center encounters a structure that acts as a planar dielectric stack reflector, also known as a Bragg mirror, which can reflect light effectively within a given range of wavelengths. It is generally considered better to have more rings, for example, 7 or 9 bilayers, to enhance the reflective properties of the waveguide and reduce losses.
FIG. 2a is a simplified diagram of a cross-structure of an exemplary Bragg fiber waveguide 200. Bragg fiber 200 consists of a core extending on the waveguide axis, normally of a low dielectric material such as air or other gas and having an index of refraction, surrounded by concentric substantially annular rings 201 having different indices of refraction, alternating to form bilayers in some embodiments. Typically a protective outer layer 202, often of a polymer material, is provided.
Recently, researchers such as Fink et al., have developed design tools and manufacturing processes to produce Bragg fibers with desired dispersive characteristics, which enable them to be used for dispersion management in optical communications systems, for example to provide very low dispersion or to compensate for unwanted dispersion during transmission of long haul telecommunication signals. Patents and publications on such developments based upon Fink's work include U.S. Pat. No. 6,603,911 by Fink et al.; U.S. Pat. No. 6,728,439 by Weisberg et al.; U.S. Pat. No. 6,788,864 by Ahmad et al.; and published U.S. patent application Ser. No. 20020176676 by Johnson et al.
Fink and others have demonstrated that light can propagate in these fibers with very low optical losses and low non-linearities, and modeling results have shown that the dispersion of these fibers can be tailored to range over several orders of magnitude. Depending on the number and configuration of the ring cores around the core of the fiber, very high dispersion parameters may be obtained in such fibers by introducing a “defect” or “aberration” in one or more of the annular rings, for example, an irregular ring thickness in the periodic annular ring distribution. FIG. 2b shows a simplified diagram of a cross-structure of an exemplary Bragg fiber waveguide 203 in which a ring 204 of an irregular thickness has been introduced. The irregular ring 204 creates a resonant mode that couples to the central mode of the fiber over certain frequency ranges. At frequencies close to the frequency at which this coupling occurs there can be large dispersions, which can be tailored by controlling the thickness, refractive index, and radial position of the irregular ring 204.